This invention relates to a transistor circuit, particularly to a transistor amplifier circuit in which changes of its input impedance due to a variation of the current amplification factor of a transistor are effectively prevented.
In general, changes of input impedance of a transistor amplifier circuit affect the whole circuit operation. For instance, in a charge/discharge circuit of a sample/hold circuit or in an amplifier circuit for amplifying a signal detected by an AM detector, output level variations due to input impedance changes cause an erroneous or inaccurate operation of the circuit. It is therefore essential to design the circuit so as to reduce the input impedance changes.
Such a problem as mentioned above will be described with reference to FIG. 1. In this figure, an amplitude-modulated signal applied to a terminal P1 is envelope-detected by the circuit comprising a base-emitter diode of transistor Q1 and a capacitor C1. The envelope-detected signal is current-amplified by transistors Q2 and Q3 and then outputted from a terminal P2 which is connected to an emitter resistor R1 of the transistor Q3. When the current amplification factors .beta.1 and .beta.2 of the transistors Q2 and Q3 are varied, the output signal level is also varied. This is because the product of the capacitance of capacitor C1, the resistance of resistor R1 and the variations of current amplification factors .beta.1, .beta.2 of transistors Q2, Q3 considerably varies the discharge time constant for the charged capacitor C1. Thus, the variation of current amplification factors of transistors Q2 and Q3 affects the operation of the whole circuitry. In other words, the input impedance of transistor Q2 as well as the discharge time constant are influenced by the product of the current amplification factor variations of transistors Q1, Q2, that is, .DELTA..beta.1, .DELTA..beta.2.
To make the circuit operate more accurately, input impedance changes of the transistor circuit should be avoided as much as possible.